[postgis-users] Dot Density idea
Martin Davis
mbdavis at refractions.net
Thu May 6 10:49:37 PDT 2010
Sounds like it could work - with maybe a bit of fiddling to deal with
cases where the grid cells overlapped the polygon only slightly?
Random perturbation by cell radius can still result in some points being
very close together. (And I think this would also be an issue where
only a small part of each grid cell overlapped the polygon). This may
or may not be desirable. Perhaps a further check could be made to
reduce the radius for points where this occurs. Or maybe some sort of
simulated annealing process could be use to push the points into a more
even distribution.
M
Paul Ramsey wrote:
> Even-yet-random :) nice requirement. How about just starting with a
> regular grid and then perturbing the elements randomly with a radius
> of a cell size? You can use the area of the polygon and number of
> needed points to calculate the appropriate cell size and go from
> there.
>
> P
>
> On Thu, May 6, 2010 at 10:28 AM, Martin Davis <mbdavis at refractions.net> wrote:
>
>> Good point about the need for even distribution of the points. That seems
>> like a whole lot harder to code than simply randomly placing points in a
>> polygon. Does anyone have any pointers to algorithms for producing this
>> effect?
>>
>> George Silva wrote:
>>
>>> The really big problem with dot density is that dots can overlap
>>> themselves,
>>> masking the real number, so if anything will be developed in this area,
>>> the
>>> points should be
>>>
>>> A) evenly distributed
>>> or
>>> B) randomly distributed, but with some sort of "colision" tests, so there
>>> is
>>> no or little overlap.
>>>
>>> This is a interesting idea, especially if we could make a materialized
>>> view
>>> with those points, which could be added to GIS software for presentation.
>>>
>>> George
>>>
>>> On Thu, May 6, 2010 at 1:53 PM, Sufficool, Stanley <
>>> ssufficool at rov.sbcounty.gov> wrote:
>>>
>>>
>>>
>>>> Looks nasty, but it might work:
>>>>
>>>> select
>>>> st_line_interpolate_point(
>>>> st_intersection(
>>>> the_geom,
>>>> st_makeline(
>>>> st_pointn(st_exteriorring(the_geom), (rand1.rand *
>>>> st_npoints(st_exteriorring(the_geom)))::int),
>>>> st_pointn(st_exteriorring(the_geom), (rand2.rand *
>>>> st_npoints(st_exteriorring(the_geom)))::int)
>>>> )
>>>> )
>>>> ,rand3.rand
>>>> )
>>>> from insert_your_table_name_here,
>>>> (select random() as rand, generate_series(1,1000) as point_number) as
>>>> rand1
>>>> JOIN (select random() as rand, generate_series(1,1000) as point_number)
>>>> as
>>>> rand2
>>>> ON rand1.point_number = rand2.point_number
>>>> JOIN (select random() as rand, generate_series(1,1000) as point_number)
>>>> as
>>>> rand3
>>>> ON rand2.point_number = rand3.point_number
>>>> WHERE st_geometrytype(
>>>> st_intersection(
>>>> the_geom,
>>>> st_makeline(
>>>> st_pointn(st_exteriorring(the_geom), (rand1.rand *
>>>> st_npoints(st_exteriorring(the_geom)))::int),
>>>> st_pointn(st_exteriorring(the_geom), (rand2.rand *
>>>> st_npoints(st_exteriorring(the_geom)))::int)
>>>> )
>>>> )
>>>> ) = 'ST_LineString'
>>>> AND oid = 5030 /* Enter your own OID here */
>>>> limit 100
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: postgis-users-bounces at postgis.refractions.net
>>>>> [mailto:postgis-users-bounces at postgis.refractions.net] On
>>>>> Behalf Of Martin Davis
>>>>> Sent: Thursday, May 06, 2010 8:56 AM
>>>>> To: John Abraham; postgis-users at postgis.refractions.net; Martin Davis
>>>>> Subject: Re: [postgis-users] Dot Density idea
>>>>>
>>>>>
>>>>> I was thinking the same thing!
>>>>>
>>>>> strk wrote:
>>>>>
>>>>>
>>>>>> ST_RandomPoinsOnSurface(geometry, numpoints) would be an interesting
>>>>>> function indeed. Sounds like a good job for GEOS/JTS.
>>>>>>
>>>>>> --strk;
>>>>>>
>>>>>> On Mon, May 03, 2010 at 10:49:32PM -0600, John Abraham wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>> One of the things I miss about using ESRI's GIS is the
>>>>>>>
>>>>>>>
>>>>> ability to do
>>>>>
>>>>>
>>>>>>> dot-density maps. Within a polygon, the number of dots is
>>>>>>>
>>>>>>>
>>>>> proportional to a value, and the dots are randomly placed. I
>>>>> find it useful to be able to present several data values at
>>>>> once (e.g. blue dots for population, red dots for employment).
>>>>>
>>>>>
>>>>>>> I also find that it is a more intuitive way of scaling for
>>>>>>>
>>>>>>>
>>>>> zone size
>>>>>
>>>>>
>>>>>>> than dividing the value by the area of the zone. That is,
>>>>>>>
>>>>>>>
>>>>> the count
>>>>>
>>>>>
>>>>>>> of the dots represents the actual number, but the density
>>>>>>>
>>>>>>>
>>>>> of the dots
>>>>>
>>>>>
>>>>>>> represents the density of the number. So I don't have to decide
>>>>>>> whether to divide the value by the area of the polygon to plot
>>>>>>> density: both the absolute number and the density are
>>>>>>>
>>>>>>>
>>>>> easily visible.
>>>>>
>>>>>
>>>>>>> Since my open-source GIS viewing systems (mostly QGIS and
>>>>>>>
>>>>>>>
>>>>> Mapserver)
>>>>>
>>>>>
>>>>>>> won't plot dot-density, I've done without.
>>>>>>>
>>>>>>> But today I realized that I can build these on the server
>>>>>>>
>>>>>>>
>>>>> instead. I
>>>>>
>>>>>
>>>>>>> can generate random points within the bounding-box of the polygon,
>>>>>>> throwing out those that aren't contained within the polygon,
>>>>>>> repeating until I have enough. Then I can save these points as a
>>>>>>> separate layer, and display this layer using almost any desktop or
>>>>>>> web based viewer!
>>>>>>>
>>>>>>> Has anyone done this? Can I do it in SQL or do I need to write
>>>>>>> something in PL/pgsql?
>>>>>>>
>>>>>>> --
>>>>>>> John Abraham
>>>>>>>
>>>>>>> PS I just bought the Postgis In Action book; enjoying it so far.
>>>>>>> _______________________________________________
>>>>>>> postgis-users mailing list postgis-users at postgis.refractions.net
>>>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>> --
>>>>> Martin Davis
>>>>> Senior Technical Architect
>>>>> Refractions Research, Inc.
>>>>> (250) 383-3022
>>>>>
>>>>> _______________________________________________
>>>>> postgis-users mailing list
>>>>> postgis-users at postgis.refractions.net
>>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>>
>>>>>
>>>>>
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>>>>
>>>
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>> --
>> Martin Davis
>> Senior Technical Architect
>> Refractions Research, Inc.
>> (250) 383-3022
>>
>> _______________________________________________
>> postgis-users mailing list
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>>
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--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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